1. Field of the Invention
The present invention relates to a vitreous silica crucible.
2. Description of the Related Art
In general, silicon single crystal is manufactured by melting high-purity polycrystalline silicon in a vitreous silica crucible to obtain a silicon melt, dipping an end of a seed crystal to the silicon melt, and pulling the seed crystal while rotating it.
The melting point of silicon is 1410 degrees C., and thus the temperature of the silicon melt is kept at a temperature equal to or higher than 1410 degrees C. At such temperature, a vitreous silica crucible reacts with the silicon melt, and the thickness of the crucible wall gradually decreases. When the thickness of the crucible wall decreases, the strength of the crucible is lowered. This leads to problems such as buckling and sidewall lowering of the crucible.
In order to solve such problems, there is known a technique to provide a layer, on the outside of the crucible, to promote crystallization. When such a layer is provided, the outer layer of the crucible is crystallized and the crucible strength is improved (e.g., JP-A-2000-247778). When such a layer is provided, the outer layer of the crucible is crystallized when the crucible is heated for a long time. Crystalline silica has higher strength per unit thickness than vitreous silica. Therefore, the crystallization enhances the strength per unit thickness, and suppresses buckling or sidewall lowering of the crucible.